Olefin polymers formed by the polymerization of olefin monomers in the presence of a Ziegler-Natta catalyst have a high degree of crystallinity and beneficial physical properties which make them particularly useful in the production of molded articles, films and fibers. Ziegler-Natta catalysts are formed by the reaction of an inorganic compound of a metal of Groups I-III of the Periodic Table, such as an aluminum trialkyl, with a compound of a transition metal of Groups IV-VIII of the Periodic Table, such as titanium tetrahalide. Olefin polymers which are both stereoregular and sterospecific are formed by the polymerization of olefin monomers in the presence of certain Ziegler-Natta type catalysts. Typically the crystallinity is from about 20 to about 90% as determined by X-ray diffraction.
Notwithstanding the very desirable and beneficial properties of these olefin polymers, they are quite susceptible to oxidative degradation due to exposure to atmospheric agents over time and to exposure to thermal procedures, particularly at the elevated temperatures used over the period of time required to process, e.g., mill, mold, extrude, and spin, the olefin polymers. Hence, a number of stabilizers have been developed over the years which tend to inhibit the oxidative degradation of these olefin polymers. However, these stabilizers suffer from one or more deficiencies. For example, they have an adverse affect on the physical properties of the olefin polymers during processing, they fail to provide a product which has any appreciable storage stability, they bloom or they provide a product which is prone to gas yellowing.
Gas yellowing occurs during the production of fibers or films from olefin polymers which have been stabilized with phenolic stabilizers or during the storage of olefin polymers, especially when stored in particulate form, such as, powder or flake. It has been shown that such gas yellowing is typically caused by nitration or nitrosation of certain hindered phenolic stabilizers in the para position with the oxides of nitrogen present in the atmosphere.
To prevent nitration or nitrosation in the para position, phenolic stabilizers having an ester group in the meta position, such as those disclosed in U.S. Pat. Nos. 3,795,700, 3,998,863 and 3,923,869, were developed. These ester groups tend to sterically or otherwise hinder nitration or nitrosation at the para position. However, these stabilizers either have minimal compatibility with olefin polymers or have minimal solubility with the solvents used in "in-process" stabilization or both.
A costabilizer system for polyolefins containing 1) a hindered phenol and 2) an ester formed by reacting a sulfur-containing aliphatic carboxylic acid with certain cyclic terpene alcohols or hydrogenated derivatives thereof, such as tetrahydroabietyl alcohol, is disclosed in U.S. Pat. No. 3,630,991.
U.S. Pat. Nos. 4,775,496 and 4,946,879 describe antidegradants for rubber compounds consisting of the reaction product of a rosin acid and a polyfunctional compound having at least one functional group capable of reaction with a carboxylic acid functionality and another functional group having antidegradant properties selected from the group consisting of 4-hydroxymethyl-2,6-di-t-butylphenol, 4,4'-methylenebis-(2,6-di-t-butylphenol), 4,4'-butylidenebis-(6-t-butyl-3-methylphenol), 4,4'-thiobis-(6-t-butyl-m-cresol), 4,4'thiobis-(6-t-butyl-o-cresol), 2-mercaptobenzimidazole, p-amino-diphenylamine, p-hydroxy-diphenylamine, p-hydroxy-p'-amine-diphenylamine and p,p'-diamino-diphenylamine.